The term “green roof” is often used to describe a roof or similar surface that has been adapted to facilitate vegetative growth. Accordingly, a green roof system will employ a vegetative system that includes growth media and, typically, features to permit water that permeates the system to drain from the roof.
Traditional green roof systems rely on aggregate particle size distribution (PSD) as the primary means of retaining storm water. The standards for PSD originate with the FLL guidelines. FLL guidelines are green roof standards developed by the German Research Society for Landscape Development and Landscape Design (also known as Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau e.V.). Generally the FLL guidelines call for an even-graded PSD to create approximately 35% void space, which is distributed between macropores and micropores. Micropores have a higher water retention capacity than macropores. Use of greater micropores requires a denser media (i.e., a greater proportion of small particle sizes), which adds weight to the system. Medias compliant with FLL guidelines weigh approximately 6-8 lbs/f2 per inch of system depth when fully saturated. These medias retain up to 0.3 inches of rain per 1-inch of thickness, an efficiency which decreases as thickness increases. For example, a 4-inch green roof with FLL-compliant media will retain approximately 1 inch of storm water, whereas doubling the thickness to 8 inches will yield approximately 1.5 inches of storage. A 4-inch thick green roof compliant with FLL guidelines weighs approximately 28 lbs/f2, of which only approximately 5 lbs, or 20% of the saturated weight, is water.
Furthermore, current vegetative systems used in green roofs may require significant maintenance to inhibit weed growth or promote growth of the desired vegetation and are inefficient storm water retention devices.